Re: [PATCH v2] mm/hugetlb.c: make huge_pte_offset() consistent and document behaviour

[Mike Kravetz]

On 08/21/2017 11:07 AM, Catalin Marinas wrote:
> On Fri, Aug 18, 2017 at 02:29:18PM -0700, Mike Kravetz wrote:
>> On 08/18/2017 07:54 AM, Punit Agrawal wrote:
>>> When walking the page tables to resolve an address that points to
>>> !p*d_present() entry, huge_pte_offset() returns inconsistent values
>>> depending on the level of page table (PUD or PMD).
>>>
>>> It returns NULL in the case of a PUD entry while in the case of a PMD
>>> entry, it returns a pointer to the page table entry.
>>>
>>> A similar inconsitency exists when handling swap entries - returns NULL
>>> for a PUD entry while a pointer to the pte_t is retured for the PMD entry.
>>>
>>> Update huge_pte_offset() to make the behaviour consistent - return a
>>> pointer to the pte_t for hugepage or swap entries. Only return NULL in
>>> instances where we have a p*d_none() entry and the size parameter
>>> doesn't match the hugepage size at this level of the page table.
>>>
>>> Document the behaviour to clarify the expected behaviour of this function.
>>> This is to set clear semantics for architecture specific implementations
>>> of huge_pte_offset().
>>>
>>> Signed-off-by: Punit Agrawal <punit.agrawal@xxxxxxx>
>>> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
>>> Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
>>> Cc: Steve Capper <steve.capper@xxxxxxx>
>>> Cc: Will Deacon <will.deacon@xxxxxxx>
>>> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
>>> Cc: Michal Hocko <mhocko@xxxxxxxx>
>>> Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
>>> ---
>>>
>>> Hi Andrew,
>>>
>>> From discussions on the arm64 implementation of huge_pte_offset()[0]
>>> we realised that there is benefit from returning a pte_t* in the case
>>> of p*d_none().
>>>
>>> The fault handling code in hugetlb_fault() can handle p*d_none()
>>> entries and saves an extra round trip to huge_pte_alloc(). Other
>>> callers of huge_pte_offset() should be ok as well.
>>
>> Yes, this change would eliminate that call to huge_pte_alloc() in
>> hugetlb_fault().  However, huge_pte_offset() is now returning a pointer
>> to a p*d_none() pte in some instances where it would have previously
>> returned NULL.  Correct?
> 
> Yes (whether it was previously the right thing to return is a different
> matter; that's what we are trying to clarify in the generic code so that
> we can have similar semantics on arm64).
> 
>> I went through the callers, and like you am fairly confident that they
>> can handle this situation.  But, returning  p*d_none() instead of NULL
>> does change the execution path in several routines such as
>> copy_hugetlb_page_range, __unmap_hugepage_range hugetlb_change_protection,
>> and follow_hugetlb_page.  If huge_pte_alloc() returns NULL to these
>> routines, they do a quick continue, exit, etc.  If they are returned
>> a pointer, they typically lock the page table(s) and then check for
>> p*d_none() before continuing, exiting, etc.  So, it appears that these
>> routines could potentially slow down a bit with this change (in the specific
>> case of p*d_none).
> 
> Arguably (well, my interpretation), it should return a NULL only if the
> entry is a table entry, potentially pointing to a next level (pmd). In
> the pud case, this means that sz < PUD_SIZE.
> 
> If the pud is a last level huge page entry (either present or !present),
> huge_pte_offset() should return the pointer to it and never NULL. If the
> entry is a swap or migration one (pte_present() == false) with the
> current code we don't even enter the corresponding checks in
> copy_hugetlb_page_range().
> 
> I also assume that the ptl __unmap_hugepage_range() is taken to avoid
> some race when the entry is a huge page (present or not). If such race
> doesn't exist, we could as well check the huge_pte_none() outside the
> locked region (which is what the current huge_pte_offset() does with
> !pud_present()).
> 
> IMHO, while the current generic huge_pte_offset() avoids some code paths
> in the functions you mentioned, the results are not always correct
> (missing swap/migration entries or potentially racy).

Thanks Catalin,

The more I look at this code and think about it, the more I like it.  As
Michal previously mentioned, changes in this area can break things in subtle
ways.  That is why I was cautious and asked for more people to look at it.
My primary concerns with these changes in this area were:
- Any potential changes in behavior.  I think this has been sufficiently
  explored.  While there may be small differences in behavior (for the
  better), this change should not introduce any bugs/breakage.
- Other arch specific implementations are not aligned with the new
  behavior.  Again, this should not cause any issues.  Punit (and I) have
  looked at the arch specific implementations for issues and found none.
  In addition, since we are not changing any of the 'calling code', no
  issues should be introduced for arch specific implementations.

I like the new semantics and did not find any issues.

Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>

-- 
Mike Kravetz